Ohila: 'And here he is at last. The man to end it all. My sisters, the Doctor has returned to Karn. We have always known in our bones that one day he would return here. Such a pity he's dead.' -From 'The Night Of The Doctor'

Ohila, from the Sisterhood of Karn, is referring to the 8th Doctor who was temporarily restored to life by the Sisterhood. Less than 3 minutes later, when he drinks an elixir (given to him by Ohila) he is changed into a 'warrior'. Onset of this change is marked by regeneration energy which begins to flow from the Doctor's body. While his outward appearance is altered we do not witness a full regeneration. Why? Because only the living regenerate... what we witnessed was rebirth from death, not regeneration of life.